The present invention relates to a variable passive element and more particularly to a pre-set variable passive element having a movable part for adjustment made of a conductive material and mounted on a board made of a conductive material.
FIG. 1 is a cross sectional view of a conventional variable passive element fitted on a board, and FIG. 2 is a partially sectioned perspective view taken along the line pointed by the arrow A of FIG. 1.
In FIGS. 1 and 2, the notation numeral 1 represents a pre-set variable resistor, which is a variable passive element. This pre-set variable resistor 1 is mounted on a stator board 3, on which a spindle motor 2 of a disk drive is formed, and is used as an electronic part for adjusting the timing of index of the spindle motor 2. This pre-set variable resistor 1 which is made of an insulating material such as a ceramic and is composed of a main part 1a, on top of which a resistor (not shown in the figure) is applied, and an adjusting member 1b made of a conducting material, in which member an electric contact 1c is slid on the resistor. With the rotation of this adjusting member 1b, the position of the contact 1c on the resistor is changed and the value of resistance changes. As shown in FIGS. 1 and 2, this adjusting member 1b forms a protuberance on the surface of main part 1a so that the rotational operation can be easily performed.
Generally, in a spindle motor for a disk drive such as the spindle motor 2, the stator board 3 often serves as a yoke of the motor because a coil 2a is disposed on the stator board 3. In this case, the stator board 3 is made of a thin steel plate. Recently, in order to meet the demand for thinner and smaller disk drive, this stator board 3 made of steel plate for the purpose of serving as the yoke is often used as a circuit board for mounting electric parts for motor driving circuit. Accordingly, the production of the stator board 3 should be such that a steel plate is first processed into a specified form, an insulating paint is applied on the surface, a specified circuit pattern of copper foil is formed thereon by a method such as etching, and a masking paint is applied, after which devices to be mounted and cream solder are disposed.
Since a disk detaching/attaching mechanism (not shown in the figure) is fitted above the spindle motor 2 in this disk drive, the adjusting of the adjusting member 1b of the pre-set variable resistor 1 is performed from underneath of the stator board 3. Therefore, when mounting the pre-set variable resistor on the stator board 3, an opening 3a is bored in the stator board 3 as shown in FIGS. 1 and 2, through which opening 3a the adjusting of the adjusting member 1b is performed from the underside 3b of the stator board 3.
In the conventional technology described above, when the position at which the pre-set variable resistor is fitted is displaced in a direction parallel with the surface of the stator board 3, the adjusting member 1b protruding from the main part 1a and an inside periphery of the opening 3a of the stator board 3 come into contact.
Since the adjusting member 1b of the pre-set variable resistor 1 has a contact by which the adjusting of the value of resistance is performed and is formed in a conductive material, the opening 3a of the stator board 3 being bored after the surface of the steel plate is treated with an insulating paint, for example, the inside periphery of the opening 3a is not treated with an insulating material. Consequently, the conventional technology has a disadvantage in that a malfunction in a motor driving circuit is caused as a result of the adjusting member 1b and the inside periphery of the opening 3a coming into contact to cause a flow of unnecessary electric current.
An idea has been brought forth that the inside periphery of the opening 3a be treated with an insulating material but the forming of a insulating film on the cross section of the thin stator board 3 is not only difficult but accompanied by an unfavorable effect of raising a cost because of the insulation treatment.